The invention relates to a method for removing a plurality of raised places (spots, points) of contact of a meltable metal, such as tin or indium or an alloy, such as tin-containing solder, silver-containing solder or lead-containing solder, said meltable metal being meltable above a first temperature limit, said places of contact being surface-distributed over a substrate. The invention also relates to a method for forming vaulted domes on a plurality of metallic support segments which are distributed over one of the surfaces of a substrate.
In order to explain the problem solved by the methods of the invention, it is noted that not only flat conductive domes provide raised places of contact—also designated as “menisci” which are applied onto metallic support segments—, but that also stronger raised metallic structures can be applied on metallic support segments, said structures being used as solder balls or contact bumps for a flip-chip contact, compare the publication “Semiautomatic Solder Ball Bumper (SB2) by Pac Tech and the publication “Solder Ball Bumper (SB2)—A flexible manufacturing tool for 3-dimensional sensor and microsystem packages”, Kasulke, Schmidt, Titerle, Bohnaker, Oppert, Zakel, published in IEMT Europe 1998, FIG. 6 and FIG. 8 in cross-section. Said raised places of contact are manufactured by several different methods, e.g. by laser or by (re-) shaping in an oven, after a preceding placement of a print of smaller solder particles which are melted in said oven, to provide the described raised places of contact in the form of balls. Usually, their height is substantially larger than that of the metallic support segments which are distributed over the substrate. A not completely avoidable probability that metallic bridges are formed by solder balls growing over to other solder balls, caused by too thick layers of flow medium, or that individual solder balls are generally missing, makes the use of the substrate technically impossible, because certain contact places are either not contacted or have a shortcut already prior to being contacted.
A further defective contact can occur due to the fact that a too large quantity of the solder forming said contact bump was applied on the substrate, thus causing an asymmetry of the formed raised contact places, said contact places possibly showing asymmetries in height as well as in a lateral extension, similar to the defects at certain, usually individual metallic support segments, as described before.
Substrates, particularly for forming wafers, are usually too precious to deny the use of the complete substrate if individual portions are damaged by applying the raised contact places upon preparing the contact. Therefore, an inexpensive solution has to be found to repair these contacted substrates which have been damaged during contact. In this respect, a specific solution has already been proposed according to the prior art, namely individually repairing individual contact places as SBB Repair Station (compare the above mentioned publication “Solder Ball Bumper (SB2)”, FIG. 10 with the associated description). One single laser impulse is used for obtaining a reflow of the defective contact point and for removing the melted and softened solder bump by using a vacuum. Simultaneously, a nitrogen atmosphere is used to avoid an oxidation of the remaining solder cap on a laterally limited support segment. A basic technical problem solved by the invention is to reduce production cost, particularly to allow a removal of a solder once applied on a substrate, when defects occur, large raised places of contact not being (selectively) located at the positions of the metallic support segments, but, by forming bridges, providing a contact structure which no longer allows a regular use of the substrate, for example as a contacted wafer.
According to the invention, several contact places can be repaired together, such that at least a substantial part of the individual contact places is removed by melting off, said melting taking place in a molten metal, and no individual places of contact have to be removed individually and successively by local heating.
According to the invention, the substrate and all contact places are contacted with a molten metal to remove the complete pre-contacting in an inexpensive and timesaving way, to start from a new status quo and to again coat the substrate with raised places of contact. Said repairing method is a removal of a preceding contact in combination with an option to start the new contact at a state, at which all metallic support segments, which continue to be present, have the same design. This concern, both the contact places not having been contacted by a raised solder ball, and the contact places undergoing a shortcut with at least one further contact place due to an inexactly positioned solder ball.
By melting off, the solder of the raised contact point is taken up by the molten metal. Domes of small height which remain on said support segments receive substantially the metal or the metal alloy of which said molten metal consists. At the same time, said option allows also for repair of defectively contacted substrates by exchanging the first solder which has already solidified, and replacing said solder by the solder corresponding to the molten metal, particularly the molten metallic alloy, which is present in the bath.
When the two metals, particularly the two metal alloys are identical, i.e. the metal of the raised places of contact and the metal of the melted bath, the substances are only removed and not exchanged. At least substantial parts of the already solidified raised places of contact on the substrate are, however, removed, the described domes remaining as “menisci” of small height.
The method is suitable for both de-bumping, i.e. removing of already existing bumps, which bumps are the raised contact places, but it is also suitable for a first time coating of support segments which have not yet been coated. Coating can be performed by applying the molten metal which is present in the bath, but coating can also be performed by removing and newly applying the mentioned molten metal.
Additionally, no more than a covering layer of an organic fluid can for example be applied on the substrate, said covering layer as a film having a small thickness with respect to the substrate. Due to the small thickness of said fluid layer (a thin layer), said layer evaporates after having contacted the molten metal due to the vapor pressure, so that a subsequent cleaning step of the support segments coated for the first time or of the support segments coated again after removal of the pre-contacting and of the associated substrate is not required. Production or repair therefore becomes less expensive. A substantially smaller volume of the fluid applied particularly as a polyalcohol, like glycerol, is used which fluid has evaporated after the described process. In general, the invention avoids the use of a bath of considerable volume with respect to the described fluid.
A first temperature limit is to be understood such that a meltable metal is to be removed, but the substrate is not to be melted. Raised contact places, which are usually made of solder, are meltable at temperatures of below 200° C. (usually below 250° C.). When using glycerol, the organic fluid used has a boiling temperature of 290° C. Similar or comparable substances should have a boiling temperature of above 250° C. When the molten bath is made of an identical metal (particularly as a metal alloy), said first temperature limit also refers to said metal which should accordingly be at least at or above said temperature limit to allow the raised contact places to be melted off and taken up from the substrate.
The distributed arrangement of the described raised contact spots or of the metallic support segments carrying said spots concerns a distribution of the basic contact over a large surface, which are in turn each provided on a metallic basic pad which allows a contact with the interconnecting paths of the substrate when provided as wafers. Said large-surface distribution is a starting point or basis of the invention for proposing a large-area removal by melting off from the substrate.
In accordance with the invention, the raised contact places can be removed substantially completely or completely, and at the same time, their material property can be exchanged.
Suitable substrates can be the substrates also described in the prior art, for example wafers, printed cards, printed boards or ceramic substrates.
As far as a metal is mentioned in the following discussion, a metal alloy is always included. This is valid for both the raised places of contact and the molten metal in the bath. As a bath both a stationary melt, which is contacted by a relative movement with the contact bumps to be removed, and a melt which is at least partly in motion and towards which the substrate with the raised contact places to be removed is moved, can be used. A lowering in a vertical direction corresponds to a practically simultaneous lateral or surface removal of all contact bumps. A relative movement in a horizontal direction in which the substrate approaches a wave-shaped or surge-shaped vertically protruding metal bath corresponds to a band-shaped removal of the raised contact places which is performed successively, but uniformly with regard to the complete surface.
The residence time for an at least contacting immersion or for an approaching movement is determined, to have sufficient time available for melting off the raised contact places and for maintaining or actually providing the remaining dome coating on the support segments.
The thin layer of the medium selected in accordance with the invention evaporates from the substrate surface substantially without residue during contacting with the molten metal. Said film can be applied by spraying or by a vapor atmosphere in the sense of sputtering. When the support segments are provided with raised contact places of a meltable metal already prior to being contacted with the molten metal, said film comes to rest as a thin fluid layer between the substrate and the support segments distributed over said substrate. When raised contact places of a meltable metal have not yet been provided on the support segments, said fluid film also covers the support segments themselves.
A vapor coating can be applied on the substrate simultaneously with the contacting of the metal bath; however, the atmosphere can extend only so far in front of the metal melt that the thin, film-like application of the organic fluid is obtained by a passage or a movement of the substrate with the support segments through said atmosphere.
Said organic fluid can satisfy the same conditions and can be made of the same materials as described in EP 781 186 B1.
The fluid covers the substrate as a covering layer in the form of a film, the film surface which faces away from said substrate directing towards the atmosphere and contacting it. Before being contacted with the molten metal, said support segments can either be covered by said organic fluid film or by said raised contact bumps to be removed; in both cases, the covering layer is formed between the substrate as well as the support segments distributed over said substrate and the surface of the bath. Said formation can also be achieved by spraying before contacting the metal melt.
The use of the fluid layer provided as a covering layer only, which due to its film shape cares for a minimum volume of the fluid to be used, can not only be applied on the substrate by spraying or sputtering, it can also be present as a thin layer on the surface of the metallic bath, or it can be formed during the metallic contact, when the evaporated organic fluid is present in the atmosphere above said bath. All embodiments used achieve a minimum volume quantity of the auxiliary processing fluid and dispensability of an additional cleaning step. Further effects of said fluid in connection with the solder to be applied or removed and with the support segments on the substrate showing a high wettability are described in detail in the prior art (uniformity of the surface structure, reducing effect on the surface, and increase of the wettability of the solder). Moreover, when removing (melting off) said raised solder spots, there is an additional advantageous aspect, namely of supporting the pinch of a reduced surface tension when melting the metal to be removed from the substrate into a molten metal which has a substantially larger volume, so that a substantially uniform formation of menisci on all limited support segments is achieved.
An example may explain the relative proportions: When a wafer has for example a thickness of 600 μm, and the support segments on the substrate have a height of between 5 μm and 10 μm, the covering layer of auxiliary fluid, which is provided as a film, has a film thickness of 50 μm to 200 μm, particularly in a range of 100 μm, said thickness also depending on the height of the raised contact places which, in a range of between 10 μm and 100 μm, can occur as ball-shaped contact bumps. This explains the definition of de-bumping according to which said contact bumps are removed from a surface of a substrate.